1. Field of the Invention
The exemplary embodiment(s) of the present invention relates to a field of linear regulator. More specifically, the exemplary embodiment(s) of the present invention relates to a programmable low dropout linear regulator using a feedback network of active load.
2. Description of Related Art
Power supplies provide necessary power consumption for the operation of electronic systems. The source of power may be a battery or a supply circuit. An unregulated power source is not able to supply power stable enough for circuits or systems having strict requirement of power supply. The unregulated power source will adversely affect the circuit or system performance and even result in malfunction, which degrades the reliability of the system or circuit. Moreover, power conversion is needed for a system or circuit with portions requiring a supply voltage having a level different from that provided by the power source. This also necessitates a voltage regulator or DC-DC converter for conversion of the unregulated supply voltage into a regulated one having a required level.
Voltage regulators are mainly categorized in switching regulators and linear regulators. Switch regulators are advantageous in having an adjustable output voltage level and high power efficiency where a large difference between the input and output voltage level exists, but disadvantageous in having large ripples and noise in the output voltage. On the contrary, in comparison with the switching regulators, the linear regulators have smaller ripples and noise, but lower power efficiency in case of large input-output voltage difference. Therefore, the linear regulators are typically used as LDOs (low dropout linear regulators) where the input-output voltage difference is limited. Conventionally, a combination of the switching and linear regulator is used in high dropout conversion, wherein the switching regulator converts the voltage level while the linear regulator performs regulation of the voltage output from the switching regulator to diminish the ripples and noise therein.
With the rising of the environmental awareness of the public, and rapid development and population of electronic products, low power consumption and high power efficiency become a critical consideration in electronic product design. Systems or circuits power supplied by batteries should be operated with a low voltage/current to reduce the power consumption and extend battery life. Even those supplied by utility power usually include circuits for power management so that they can be operated with low voltage in saving or standby mode when being idle for a period of time. Moreover, with the development of nano-CMOS manufacturing technologies, the operating voltages of integrated circuits are decreasing. Thus, modern system or circuits should be usually designed to operate with a low operating voltage. Circuits operating in a low voltage have a strict requirement of power supply in order to perform adequately, and accordingly the linear regulator is a key component in a low voltage system. The advantages of an LDO include:    (1) low noise and ripple in the output voltage;    (2) better transient response to changes of the load current and input voltage;    (3) low EMI;    (4) low static current, low power consumption and high power efficiency;    (5) simple circuitry and small circuit area; and    (6) no discrete inductor used, which helps to reduce an area of the system board and product cost.
The advantages mentioned above are basic requirements of a system with a low power consumption, low voltage and low cost. Additionally, to reduce the power consumption more effectively, the functional block in a SOC (system on chip) may have multiple operation modes using different operating voltages, which is a kind of circuit design so called “Multi-Voltage Domain” and necessitates a multi-level power supply. Moreover, in consideration of both system performance and power consumption, the SOC always includes a power management mechanism able to alter the operating voltage or even turn off the power supply, depending on the requirements of the operation modes and performance. In such a case, a programmable DC power supply is necessary for the system to meet the voltage specifications in different operation modes.
Although a switching regulator is inherently a programmable DC power supply, due to its disadvantages mentioned above, the simplest and most straightforward implementation of a programmable DC power supply for a system with a low power consumption, low voltage and low cost is the combination of multiple LDOs with a multiplexer selecting a desired output from those of the LDOs as shown in FIG. 1A. Alternatively, a single LDO using multiple reference voltage generators to generate output voltages with multiple levels may be also appropriate, as shown in FIG. 1B. However, any one of the circuits shown in FIGS. 1A and 1B will occupy a relatively large chip area.
Alternatively, in order to reduce the circuit area, a programmable reference voltage generator may be used, as shown in FIG. 2. However, the circuit complexity and accuracy issue of the programmable reference voltage generator, and a high common mode voltage level of the error amplifier resulting from the alteration of the reference voltage increases the difficulty of circuit design.
There have been some studies proposing to have different output voltage levels by altering the resistance of the feedback network, as shown in FIG. 3. The relationship between the levels of the output and input voltages can be indicated by:Vout=Vref(1+R1/R2)  (1)The desired output voltage level can be obtained by changing the ratio of R1 to R2. However, in case that a large number of output voltage levels are required, a large number of resistors are necessary. Although the resistors may be implemented by discrete resistors to diminish the impact of inconsistency of process parameters and temperature dependency, such an implementation can not meet the requirement of an embedded power management and departs from the SOC design. This necessitates programmable resistor strings integrated on a single chip. The programmable resistor strings will include a large number of resistors which occupy a large circuit area and therefore increase the cost. The circuit area of the programmable resistor string may be even larger than that of an LDO.